One in every three children is overweight or obese. Overweight and obesity in childhood is associated with negative health and psychosocial consequences for children and adults. To date, the most successful weight loss treatment for children who are overweight or obese are family-based behavioral programs (FBT) that are delivered in separate parent and child groups over a 6-month period. FBT is fairly intensive and access to these programs can be challenging. We have developed a guided self-help treatment which is based on FBT (gshFBT), and provides a self-help manual to parents and children along with guidance in the form of 20-minute check-in visits every other week. Our studies show that gshFBT is feasible and acceptable to families, decreases body weight in children and parents compared to a delayed treatment control, and that it produces similar weight losses compared to the more intensive FBT. However, all of our studies with gshFBT were conducted with children who were 85%BMI-98%BMI and to date, this method has not been tested with heavier children. Additionally, our pilot studies only included a 6-month follow-up timepoint and longer follow-up is needed. Thus, the objective of this study is to conduct a fully powered randomized controlled trial comparing gshFBT with FBT in children across the overweight and obese weight spectrum. We will recruit, randomize and treat 160 overweight/obese children and their parents and assess them at baseline, post-treatment, and 6- and 18-months follow-up. Primary and secondary aims are as follows. Primary Aim 1: Compare the effect of gshFBT and FBT on the target child's weight over the two years of the study. Primary Aim 2: Evaluate the cost effectiveness of gshFBT as compared to FBT. Secondary Aim 1: Determine the extent to which the gshFBT and FBT affects parent BMI, parenting skills, and parent and child physical activity and dietary intake. Additionally, as exploratory aims, we will evaluate moderators and mediators of gshFBT and FBT including executive functioning and parenting style. This application is a critical next step in the development of treatments for overweight and obese children, and could provide a treatment that is less intensive, less costly and has the potential to be provided to a larger proportion of the population.